Method of evaluating silicon wafers

ABSTRACT

The method of evaluating silicon wafers according to this invention is capable of predicating degradation of the quality of oxide film insulation, which is incurred, on the silicon wafers, by process faults or local residual strains undetectable by the naked eye. The method includes the following steps of: removing selectively a surface of a silicon wafer treated by mirror polishing by using an etching selectivity caused by an unordinary surface state; counting the number of etch pits on the surface of the silicon wafer with the aid of an optical microscope; and judging the quality of the silicon wafer based on the etch pit density, which is calculated from the above number of etch pits, and the threshold value of etch pit density. The threshold value of etch pit density of the silicon wafer treated by selective etching is set to be below 5×10 5  pits/cm 2 , and improvements to the processing of production lines relating to low-quality silicon wafers can be made.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of evaluating silicon wafers,particularly to a method adapted to evaluate the quality of mirrorpolishing of silicon wafers and the quality of the grinding process forsilicon wafers, which is an important factor affecting the quality ofsilicon wafers.

2. Description of Prior Art

Silicon wafers employed in semiconductor devices are sliced from siliconsingle crystals, which are usually manufactured by the CZ or FZ method.The outer peripheries of the sliced silicon wafers are chamfered, thenprocesses such as grinding, lapping, and polishing are performed on bothof the front and rear surfaces so as to mirror-finish at least onesurface.

Grinding and polishing semiconductor wafers to be mirror-finishedproduces process faults (hereinafter referred as defects) such asscratching capable of being detected by eyesight inspection and localresidual strains undetectable by the naked eye. According to JP-A8-70009 (JP-A: Japanese unexamined Patent Publication) entitled "METHODOF MANUFACTURING SEMICONDUCTOR SILICON WAFERS", the existence ofresidual strains undetectable by the naked eye can be recognized byusing a microscope if selective etching is performed on a silicon wafer.Furthermore, the above residual strains can be removed by applying heattreatment to silicon wafers within a short time period at lowtemperature.

The above method of manufacturing semiconductor silicon wafers isadaptable for removing residual strains induced during mirror polishingon surfaces of silicon wafers, the result can be recognized byperforming selective etching. However, even if selective etching hasbeen performed, it is impossible to predict the extent to which therecognized defects will adversely affect the important qualities ofsemiconductor devices, for example the electrical characteristic such asoxide film insulation. Therefore, it is difficult to isolate thequality-control problems in the processing of silicon wafers and improvethem.

SUMMARY OF THE INVENTION

In view of the above drawbacks, the object of the present invention isto provide a method of evaluating silicon wafers, which can contributeto the management and improvement of the processing of silicon wafers.The method of evaluating silicon wafers is capable of easily evaluatingthe relationship between the quality of oxide film insulation and theproceeding steps such as mirror polishing and washing.

A first aspect of the method of evaluating silicon wafers is a methodaccording to this invention, which comprises the following steps of:

removing selectively a surface of a silicon wafer treated by mirrorpolishing by using a etching selectivity caused by unordinary surfacestate;

counting the number of etch pits on the surface of the silicon wafer bythe aid of an optical microscope; and

judging the quality of the silicon wafer based on the etch pit density,which is calculated from the above number of etch pits, and thethreshold value of etch pit density.

A third aspect of the method of evaluating silicon wafers is a methodaccording to the first aspect, wherein the step of removing is conductedafter mirror polishing and washing the surface of a silicon wafer.

A fourth aspect of the method of evaluating silicon wafers is a methodaccording to the first aspect, wherein the step of judging the qualityof the silicon wafer comprises a step of judging the quality to be goodin the case that the threshold value of etch pit density of the siliconwafer treated by selective etching is less than 5×10⁵ pits/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thesubsequent detailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a block diagram showing the sequence of the steps ofevaluating silicon wafers;

FIG. 2 is a graph showing the relationship between the etch pit densityof a silicon wafer and the insulation strength of an oxide film; and

FIG. 3 represent a list of etching solutions used in the selectiveetching process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

After removing a layer of depth 0.5-5 μm by performing selective etchingon the surface of a silicon wafer, the defects on the surface of thesilicon wafer are capable of being enlarged to a size visible by the aidof an optical microscope. "Selective etching" used in the presentinvention is based on the steps of oxidizing a silicon wafer and beingsolved by hydrogen oxide. In the selective etching process, an oxidationrate caused by the oxidation agent in the defective region which has acrystal defect or strain is distinct from that in the complete crystalregion (non-defective region). Namely, oxide formed in the defectiveregion is usually etched preferably to be a concave shaped pit. On theother hand the defective region where etching rate gets slower, isetched to be convex shaped mount.

In the case that the step of etching is accompanied with plurality ofreactions, etching rate are determined by the ratio of the etching agent(HF etc.) reaching to the surface of the crystal rather than the surfacestate of the crystal. Therefore by stirring an etching solution, theetching rate can be increased and the etching reaction advancesuniformly. Alternatively, by adding the buffer agent (for example H₂ O,CH₃ COOH), the etching rate can be restrained. As the oxidizing agent isincreased, the etched surface is apt to be made dim.

Therefore, etch pit density can be easily calculated, if the number ofthe etch pits appearing on the surface of the silicon wafer treated byselective etching is counted by the aid of the optical microscope.

An etch pit density obtained by following the above sequence wascompared with a threshold value of etch pit density to judge the qualityof processing the silicon wafer. It was found that only silicon waferswith etch pit density below 5×10⁵ pits/cm² have an oxide film insulationlarger than 8 MV/cm. Therefore, it is possible to properly judge whetherthe oxide film insulation satisfies the requirement of semiconductordevices or not. Then, improvements can be made to the silicon wafersjudged as bad ones.

The following is a description of an embodiment of the method ofevaluating silicon wafers, according to this invention with referencesmade to the drawings. FIG. 1 is a block diagram showing the sequence ofthe steps of evaluating silicon wafers, wherein the numerals shown onthe left side of each step denote the step numbers.

After performing mirror polishing, the surface of the silicon wafer waswashed. Then, in the first step, the silicon wafer to be evaluated wasetched to a depth of 0.5-5 μm by employing a selective etching solutionconsisting of HF: HNO₃ : CH₃ COOH: H₂ O=1: 15: 3: X (wherein the volumeratio X is usually set to be 3). At the beginning of etching, thetemperature of the selective etching solution was set to be 20-25° C.Then, in the second step, the surface of the silicon wafer etched by theabove selective etching solution was inspected by the aid of an opticalmicroscope, and the number of the etch pits existing on the surface ofthe silicon wafer was counted. In the third step, etch pit density wascalculated, based on the number of the etch pits counted, and the etchpit density obtained was compared with a threshold value to judge thequality of the silicon wafer. On this occasion, the threshold value ofetch pit density was set to be below 5×10⁵ pits/cm². Next, siliconwafers judged as bad according to the above procedure were evaluatedwith respect to their fabrication conditions, and improvement was made,based on the above evaluation results.

To examine whether the threshold value of etch pit density of thisinvention is proper or not, the following experiment was carried out. Asilicon wafer was heat-treated within an oxidation atmosphere so as toform a thermal oxide film on the wafer surface. The above silicon waferwas sliced under the same condition and was sliced from the same crystalas the silicon wafer whose etch pit density had been calculated by theaid of the optical microscope. Next, a polycrystalline silicon layer wasformed on the above thermal oxide film by the CVD method. Then, a presetnumber of polycrystalline electrodes having a predetermined size wereformed by photolithography technology. Through this procedure, MOScapacitors were constructed by disposing an insulation layer of thermaloxide film between the silicon wafer and the polycrystalline electrodes.Subsequently, an electrical voltage was applied between the siliconwafer and the polycrystalline electrodes so as to measure the insulationstrength of the oxide film.

FIG. 2 shows the relationship between the etch pit density of thesilicon wafer and the insulation strength of the oxide film in the aboveMOS capacitor structures. In FIG. 2, the symbols ▪, , ▴, respectivelydenote silicon wafers mirror polished under polishing conditions A, B,and C. Furthermore, the symbol "O" denotes the wafers proceeded under Bpolishing condition and already being treated to change its surfacestate. Based on the results of measuring the insulation strength, theabove wafers were classified into two groups. In other words, they wereclassified into a high-quality group having insulation strengths largerthan 10 MV/cm and a poor-quality group having insulation strengthsranging from 2 to 6 MV/cm. The etch pit densities of the high-qualitygroup were located to the left side of a quality judgment line, in otherwords, the values of their insulation strength were less than 5×10⁵pits/cm². The etch pit densities of the poor-quality group were locatedto the right side of the quality judgment line, namely, the values oftheir insulation strength were larger than 5×10⁵ pits/cm². The abovemeasurement confirmed that wafers having etch pit densities less than5×10⁵ pits/cm² have good insulation.

As above described, conventionally, the influence of mirror polishing ofsilicon wafers on the insulation was difficult to predict. However,according to this invention, the quality of the mirror polishing ofsilicon wafers can be easily evaluated by only measuring etch pitdensities of the wafers subjected to mirror polishing and selectiveetching. Therefore, it is possible to evaluate the quality ofproceedings without performing the evaluation of oxide film insulation,and it is possible to manage and improve the processing of siliconwafers.

As an etching solution used in a selective etching process, etchingsolutions as shown in FIG. 3 can be used. It is preferable to use asolution without containing Chromium in the view of a problem oftreatment of waste liquid.

Furthermore, the evaluation method of this invention is not limited tothe evaluation of the mirror polishing of silicon wafers. It also can beused as a simple substitute procedure for evaluating the electriccharacteristics of a silicon wafer.

What is claimed is:
 1. A method of evaluating silicon wafers comprisingthe steps of:mirror polishing and washing a surface of a silicon wafer,on which a gate oxide insulation film is to be formed; removingselectively a portion of the surface of the silicon wafer through aselective etching process that highlights the defective regions of thesurface; counting the number of defects on the surface using an opticalmicroscope; and evaluating the quality of the oxide insulation film tobe formed based on the total number of defects on the surface and athreshold value of defects.
 2. A method as claimed in claim 1, whereinthe step of removing selectively a portion of the surface of the siliconwafer uses a selective etching solution comprising HF, HNO₃, CH₃ COOH,and H₂ O.
 3. A method of evaluating silicon wafers as claimed in claim1, wherein the step of removing comprising the steps of:oxidizing saidsurface with an oxidation agent, wherein an oxidation rate of a defectregion of said surface, caused by said agent, is distinct from anoxidation rate of a non-defect region of said surface, where said defectregion is a region of said surface having crystal defects or strain; andselectively etching away a portion of said oxidized surface formed insaid oxidizing step, with an etching agent.
 4. A method of evaluatingsilicon wafers as claimed in claim 3, wherein said etching agent is HF.5. A method of evaluating silicon wafers as claimed in claim 3, whereinsaid oxidizing step and said etching step are performed simultaneouslyin a solution comprising HF, HNO₃, CH₃ COOH, and H₂ O.